Heat stabilized flame retardant styrenic polymer foam compositions

ABSTRACT

Low density, heat stabilized, flame retardant styrenic polymer foam compositions include a halogen-based flame retardant such as hexabromocyclododecane and zeolite A heat stabilizer.

This invention relates generally to heat stabilized, flame retardantstyrenic polymers and more particularly to styrenic polymer foamcompositions which include halogen-based flame retardants and zeolite Aheat stabilizers.

Halogen containing materials, for example, cycloaliphatic organiccompounds such as hexabromocyclododecane (HBCD) are widely used instyrene-based polymer compositions to impart flame retardant propertiesto the compositions. However, their presence in the styrenicpolymer-based compositions has the drawback of lowering thermalstability. This can cause serious color and/or viscosity problems whenthe compositions are exposed to high processing temperatures. In orderto improve their thermal stability, it is customary to add a stabilizingagent, such as hydrotalcite, tetrasodium pyrophosphate or dibutyl tinmaleate.

However, it has been found that when forming styrenic polymer foams, theflame retardants with normal heat stabilizers tend to increase foamdensity. It has now been found that when zeolite A is added to flameretarded styrenic polymer compositions they are not only exceptionallyheat stable even when subjected to multiple heatings, but the zeolite Ahas less effect on the foam density.

In accordance with this invention, there is provided a styrenic polymerfoam formed from the components which comprise:

(a) a styrenic polymer,

(b) a flame retardant amount of a halogen-based flame retardant,

(c) a heat stabilizing amount of zeolite A, and

(d) a blowing agent.

Also provided is a process for manufacturing an extruded, styrenicpolymer foam comprising the steps of (i) expressing a heat-plastifiedstyrenic polymer gel composition from a die so that the expressed gelexpands into cellular foam, said composition including, in addition tothe polymer, a blowing agent, a halogen-based flame retardant materialand a heat stabilizing amount of zeolite A, and (ii) cooling theexpanded cellular polymer foam to a temperature at which said foam isself-supporting.

Styrenic polymers are usually classified as general-purpose polystyrene(GPPS) or as impact-modified polystyrene (IPS). GPPS is a high molecularweight, clear polymer which is hard, rigid and free of odor and taste.It finds use in producing moldings and extrusions, including foams andfilms. IPS is a rubber-modified polystyrene which is characterized byits toughness and resistance to abuse. The rubber, such as a butadienerubber, is dispersed in the polystyrene matrix in the form of discreteparticles. IPS is not clear, but rather is either translucent or opaquedepending upon the amount of rubber used. The art recognizes two typesof IPS, i.e., medium-impact polystyrene (MIPS) and high-impactpolystyrene (HIPS), the former containing less rubber than the latter.HIPS can be generally characterized as having about an 8 to about 18 wt% rubber content. In some instances, mixtures of IPS and GPPS are usedto achieve certain blends of properties.

For the purposes of this invention, the styrenic polymer can be GPPS, ora mixture of GPPS and IPS. The GPPS and IPS may be homopolymers,copolymers or block polymers and are formed from such vinyl aromaticmonomers as styrene, ring-substituted methyl or polymethylstyrenes,ring-substituted ethyl or polyethylstyrenes, ring-substituted propyl orpolypropylstyrenes, ring-substituted butyl or polybutyl styrenes,ring-substituted mixed polyalkylstyrenes wherein the alkyl groups differfrom each other, alpha-methylstyrene, ring-substituted methyl- orpolymethyl-alpha-methylstyrenes, propyl- orpolypropyl-alpha-methyl-styrenes, butyl- orpolybutyl-alpha-methylstyrenes, ring-substituted mixedpolyalkyl-alpha-methylstyrenes wherein the alkyl groups differ from eachother, ring-substituted alkyl- or polyalkylchlorostyrenes in which thealkyl group(s) contain(s) from one to four carbon atoms, and similarpolymerizable styrenic monomers--i.e., styrenic compounds capable ofbeing polymerized by means of peroxide or like catalysts intothermoplastic resins. Homopolymers and copolymers of simple styrenicmonomers (e.g., styrene, p-methyl-styrene, 2,4-dimethylstyrene,alpha-methylstyrene, p-chloro-styrene, etc.) are preferred from thestandpoints of cost and availability.

The halogen-based flame retardants used in this invention may be anysuch flame retardants that are commonly used in this field and which aresubject to heat stability problems such as aliphatic, cycloaliphatic,and mixed-aromatic halogen compounds in which the aliphatic groupscontain halogen. Examples that may be cited include tetrabromoethane,tetrabromobutane, hexabromocyclododecane, acetylene tetrabromide,pentabromochlorocyclohexane, ethylene bis(dibromobomane dicarboximide)(BN 451), dibromoethyldibromocyclohexane (BCL 462),tetrabromocyclooctane (BC-48), melamine hydrobromide,tris(2,3-dibromopropyl)isocyanurate, tetrabromobisphenol Abis-(2,3-dibromopropyl ether), 2,3-dibromopropylpentabromophenyl ether,tetrabromophthalic anhydride and esters thereof, including RB-79 andPHT-4 diol, chlorinated polyethylenes, chlorinated paraffins, andchlorendic anhydride and derivatives thereof and the like. There is noparticular limit on the amount in which these halogen-based flameretardants are added, it being suitable to vary the amount asappropriate according to the desired degree of flame retardation. It isgenerally preferable to use 0.5-35 parts by weight, per 100 parts byweight of styrene-based resin, of one of these flame retardants alone orof two or more together. A preferred flame retardant is ahexabromocyclododecane material. This material is a mixture of isomers.

Both the low-melt and high-melt hexabromocyclododecane products havingindividual melting point ranges within the general range of about 170°C.-200° C. can be used. A most highly preferred product is HBCD-LM flameretardant available from Albemarle Corporation. This HBCD material has amelting point range of 178° C.-188° C. and an minimum melt point of 175°C.

The amount of flame retardant used is that amount which will render thefoams flame retardant. For the purposes of this invention, the term"flame retardant" is to mean that the formulation, when tested in aSteiner Tunnel in accordance with UL 723 and ASTM E-84, obtains aStandard Building Code rating of at least C (flame spread index of76-200, smoke density≦450) and preferably a rating of A or B (A=flamespread index of 0-25, B=flame spread index of 26-75). For HBCD and othercycloaliphatic halogen-based flame retardants, from about 0.5 to 8 wt %is generally used based upon the total weight of the formulation.Highest ratings can be obtained using from about 1.0 wt % HBCD at a foamthickness of 0.5" up to about 3.0 wt % at a foam thickness of 2.0".

The zeolite A used in the practice of this invention can be representedby the generalized formula for zeolite, M_(2/n) O.Al₂ O₃.ySiO₂.wH.sub.2O, wherein M is a group IA or IIA element, such as sodium, potassium,magnesium and calcium. For a sodium zeolite, the formula is Na₂.OAl₂O₃.xSiO₂.yH₂ O. The value of x normally falls within the range of1.85±0.5. The value for y can also be variant and can be any value up toabout 6. On average, the value of y will be about 5.1. For a sodiumzeolite A the formula can be written as 1.0±0.2Na₂O.AlO₃.1.85±0.5SiO2.yH₂ O, wherein the value of y can be up to about 6.An ideal zeolite A has the following formula, (NaAlSiO₄)₁₂.27H₂ O.Zeolite A is commercially available and can be purchased from AlbemarleCorporation under the trademark EZA.

The amount of zeolite A used is that amount which effects thermalstabilization of the formulation. Generally, for most formulations ofthe invention, the amount of zeolite A used will be within the range offrom about 0.1 to about 5 wt % based upon the total weight of theformulation. A preferred amount is within the range of from about 0.6 toabout 1.5 wt %.

In addition to the polystyrenic polymer, halogen-based flame retardant,and zeolite A, there can be present in the formulation conventionaladditives in their conventional amounts. Exemplary of such additivesare: fillers, pigments, dyes, impact modifiers, UV stabilizers,antioxidants, processing aids, nucleating agents, lubricants and thelike.

Transition metal-containing compounds, for example, lubricants,nucleating agents, dyes and pigments are commonly used in the styrenicpolymer compositions in amounts of from about 0.005 to 1.0 weightpercent or more of the composition. Non-limiting examples of thesecompounds include lubricants such as zinc stearate and other Zn, Cu, Fe,etc., salts of fatty acids such as stearic, tallow, coco fatty acids andthe dimer of oleic acid. Aryl carboxylate and sulfonate salts, i.e.,benzoate or terephthalate salts are used as nucleators. These compoundstend to cause serious degradation problems upon the achievement of hightemperatures and/or undergoing a heat history (masterbatch heatexperience+processing heat experience or other multiple heatingprocesses such as scrap recycle). This can occur not only when thetransition metal compound is present in functional (i.e., lubricating,nucleating, or colorant) amounts, where the transition metal is presentin amounts of from about 100 to 1,000 ppm or more by weight of polymercomposition, but even when the transition metal is only incidentallypresent in amounts of less than 100 ppm (as little as 10 ppm) by weightof the composition as a result, for example, of incorporating polymerscrap into the composition. The presence of the zeolite A stabilizerrenders the compositions exceptionally heat stable when they aresubjected to such multiple heatings.

All of the constituents are blended in any conventional manner and canbe blended in any order. For example, the constituents can first be drymixed and then fed to a Banbury mixer or twin screw extruder to obtain ablended material for feed, for example, to an injection moldingapparatus. Blending temperatures will be within the range of from about180° to 200° C.

A convenient way to add the flame retardant and stabilizer to thestyrenic polymer is as a masterbatch, which is a concentrated, heatblended or extruded mixture of the various additives in the polymer. Theconcentration of additives usually ranges from about 10 to 90 percent byweight of the total weight of masterbatch composition, with the balancebeing polymer. The masterbatch is then added to the bulk of the styrenicpolymer material, which may already contain other additives such as azinc stearate lubricant. The masterbatch is added in proportions to givethe desired concentration of additives in the final blended product.

Styrenic foam materials, for example, rods or rectangular boards, areformed, as is known, by mixing the additives, either individually or asa masterbatch, with the polymer and then feeding the mixture to anextruder along with a foaming agent and, optionally, a nucleating agent,such as talc or commercially available carbonate-based materials, forexample, the material sold under the trademark, Safoam--FP.

Any of a wide variety of known foaming agents or blowing agents can beused in producing the expanded or foamed flame resistant polymers ofthis invention. U.S. Pat. No. 3,960,792 gives a listing of some suitablematerials. Generally speaking, volatile carbon-containing chemicalsubstances are the most widely used for this purpose. They include, forexample, such materials as aliphatic hydrocarbons including ethane,ethylene, propane, propylene, butane, butylene, isobutane, pentane,neopentane, isopentane, hexane, heptane and mixtures thereof; volatilehalocarbons and/or halohydrocarbons, such as methyl chloride,chlorofluoromethane, bromochlorodifluoromethane, 1,1,1-trifluoroethane,1,1,1,2-tetrafluoroethane, dichlorofluoromethane,dichlorodifluoromethane, chlorotrifluoromethane, trichlorofluoromethane,sym-tetrachlorodifluoroethane, 1,2,2-trichloro-1,1,2-trifluoroethane,sym-dichlorotetrafluoroethane; volatile tetraalkylsilanes, such astetramethylsilane, ethyltrimethylsilane, isopropyltrimethylsilane, andn-propyltrimethylsilane; and mixtures of such material. One preferredfluorine-containing blowing agent is 1,1-difluoroethane also known asHFC-152a (FORMACEL Z-2, E. I. dupont de Nemours and Co.) because of itsreported desirable ecological properties. Water-containing vegetablematter such as finely-divided corn cob can also be used as blowingagents. As described in U.S. Pat. No. 4,559,367, such vegetable mattercan also serve as fillers. Use of carbon dioxide as a foaming agent, orat least a component of the blowing agent, is particularly preferredbecause of its innocuous nature vis-a-vis the environment and its lowcost. Methods of using carbon dioxide as a blowing agent are described,for example, in U.S. Pat. No. 5,006,566 wherein the blowing agent is 80to 100% by weight of carbon dioxide and from 0 to 20% by weight of oneor more halohydrocarbons or hydrocarbons that are gaseous at roomtemperature, in U.S. Pat. Nos. 5,189,071 and 5,189,072 wherein apreferred blowing agent is carbon dioxide and1-chloro-1,1-difluoroethane in weight ratios of 5/95 to 50/50, and inU.S. Pat. No. 5,380,767 wherein preferred blowing agents comprisecombinations of water and carbon dioxide. Such materials can be utilizedwith appropriate flame retarded styrenic polymers of this invention. Theentire teachings of the six U.S. patents whose numbers are recited inthis paragraph are incorporated herein by reference.

The invention is further illustrated by, but is not intended to belimited to, the following examples.

Masterbatch Formation

EXAMPLE 1

A masterbatch of polystyrene (Styron® 685D GPPS, Dow Chemical Co.),which polystyrene contained about 136 ppm by weight of zinc as zincstearate lubricant, was formed by blending, at a temperature of 150°C.-180° C., with mechanical mixing (100 rpm) in a Werner and PfleidererZSK 30 twin-screw, co-rotating extruder, 77 parts by weight ofpolystyrene with 23 parts by weight of a mixture containing about 75 wt% HBCD-LM flame retardant and 25 wt % zeolite A. The polymer and flameretardant were gravimetrically fed from two separate feeders. Barrelzone temperatures were 150°, 160°, 170°, 175°, and 180° C. and thethroughput was about 6 Kg/hour. The extruded strand was pelletized inline.

Comparative Example

A masterbatch was prepared using the same polystyrene and flameretardant, but with dibutyl tin maleate and2,2'-oxamidobisethyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate(Naugard XL-1) as the stabilizers. The proportions were 76 wt %polystyrene and 24 wt % of a mixture containing 94 wt % HBCD-LM, 4 wt %dibutyl maleate, and 2 wt % Naugard XL-1 stabilizer.

EXAMPLES 2-5

A 1.25 inch segmented single screw extruder having a 40/1 length todiameter ratio and a rod die was used for foaming several mixtures ofthe masterbatch prepared in Example 1 with the same type of GPPSpolystyrene used to make the masterbatch. The screw was designed tooperate in three stages: a plasticization section, a gas injectionsection, and a metering and mixing section. Several samples wereprepared to provide different concentrations of additives in thepolymer. A small amount (0.05 wt % of composition) of Safoam--FPnucleating agent was also dry blended with the mixture. The mixtureswere metered through a single screw feeder. CO₂ gas was used as aphysical blowing agent for foaming the polystyrene blends. A descriptionof the samples is given in Table 1 and the melt temperatures and CO₂ gasinjection pressures are given in Table 2. The density of the foamproducts was also measured by the water displacement technique onsamples of the product and the results are reported in Table 1. Thelowest density product was obtained at a gas injection pressure of about800-820 psi, a melt temperature of about 290° F.-300° F. and at meltpressures above 1400 psi. Temperatures at the different barrel zoneswere typically, in degrees F., 100-300-350-350-330-300-300-300-300-290and 286-292 (melt). As a comparison, several samples of foam were madein the same way but using the masterbatch composition from theComparative Example.

It was observed that the comparison samples appeared dark when extrudedat a high temperature (400° F.). The use of a lower processingtemperature was needed to reduce the color formation. In contrast, thefoam samples prepared from the composition of the invention had muchless color, even at high processing temperatures. It appears then, thateven though the tin maleate and Naugard XL-1 stabilizers were adequateto reduce decomposition during masterbatch formation, significantdecomposition occurred as a result of the second heating step when themasterbatch was used in the foaming process. The compositions of theinvention were stable during the second heating step even at highprocessing temperatures, despite the presence of the zinc stearatelubricant which otherwise would accelerate the decomposition of thebromine containing styrenic polymer composition.

                  TABLE 1                                                         ______________________________________                                                Wt %                                                                          HBCD-    Wt % Br  Fresh Foam                                                                             Aged Foam                                  Example Zeolite  Theory   Density g/cc                                                                           Density g/cc*                              ______________________________________                                        Control 0.00     0.00     0.10     0.072                                      2A      0.50     0.28     0.10     0.083                                      2B      0.50     0.28     0.10     0.075                                      3       0.67     0.38     0.11     0.075                                      4       2.00     1.12     0.11     0.078                                      ______________________________________                                                   Wt %                                                                          HBCD-Tin Maleate-                                                  Comparison Naugard XL-1                                                       ______________________________________                                        1          0.50             0.35   0.10                                       2          1.50             1.05   0.12                                       3          2.00             1.40   0.14                                       ______________________________________                                         *Measurements made 8-10 weeks after manufacture of the samples           

                  TABLE 2                                                         ______________________________________                                                                  CO.sub.2 Injection                                  Example      Melt Temp (°F.)                                                                     Pressure (psi)                                      ______________________________________                                        Control      286          813                                                 2A           288          821                                                 2B           290          804                                                 3            292          820                                                 4            289          823                                                 Comparison                                                                    1            290          816                                                 2            287          813                                                 3            286          824                                                 ______________________________________                                    

As illustrated by the foam density data in Table 1, in addition to beingmore heat stable than the comparative materials, the combination ofadditives used in Example 4, at the higher bromine level required toobtain adequate flame retardancy, such as when forming 2 to 3 inch thickbuilding insulation panels, also had less effect on the foam density. Asignificant increase in foam density was observed at a bromine level of1.4% in Comparison 3 when using the tin maleate and Naugard XL-1stabilizers. In contrast, the foam density of the samples made accordingto the invention remained about the same (within 10-15%) with increasingbromine levels. These samples also had improved color and thermalstability.

What is claimed is:
 1. A process for manufacturing an extruded, styrenicpolymer foam comprising the steps of:(i) expressing a styrenic polymergel composition fron a die so that the expressed gel expands into acellular foam, said gel composition including, in addition to saidstyrenic polymer, a blowing agent, a flame retardant amount of ahalogen-based flame retardant material, and a heat stabilizing amount ofzeolite A, and (ii) cooling the expanded polymer foam to a temperatureat which said foam is self-supporting.
 2. The process according to claim1 wherein said gel composition is made by adding a masterbatch to saidstyrenic polymer, said masterbatch having been prepared by heat blendingsaid flame retardant material and said zeolite A with styrenic polymer.3. The process according to claim 1 wherein said flame retardantmaterial is a cycloaliphatic organic bromine compound.
 4. The processaccording to claim 3 wherein said organic bromine compound is HBCD andsaid blowing agent is carbon dioxide.
 5. The process according to claim1 wherein said styrenic polymer gel composition includes a transitionmetal compound.
 6. The process according to claim 5 wherein saidcompound is a transition metal salt of a fatty acid.
 7. The processaccording to claim 6 wherein said compound is zinc stearate.
 8. Theprocess according to claim 2 wherein said masterbatch includes atransition metal salt of a fatty acid.
 9. The process according to claim8 wherein said salt is zinc stearate.
 10. The process according to claim1 wherein said polymer foam has a foam density which is substantiallythe same as a comparable polymer foam made under the same processconditions and containing the same components in the same proportions,except not including components b and c.
 11. The process according toclaim 10 wherein said foam density is no more than about 15% greaterthan the foam density of said comparable polymer foam.